1. Field of the Invention
The present invention relates to a method and an apparatus for producing a rubber hose reinforced with fiber yarn or cord, and more particularly to improved method and apparatus suitable for producing such a fiber-reinforced rubber hose which has an improved bonding strength between the reinforcing fiber layer and inner and outer rubber layers or tubes.
2. Discussion of the Prior Art
In the art of producing a rubber hose which does not have a reinforcing fiber or metal layer, it is known to practice a method wherein a microwave heating device is used for internally heating an extruded unvulcanized rubber tube and thereby vulcanizing the unvulcanized rubber material, in an effort to reduce a time required for the vulcanization.
However, the above method suffers from a drawback that the microwave heating of the unvulcanized rubber material is difficult to maintain the temperature of the rubber material within a suitable vulcanizing range. Accordingly, it is necessary to run the extruded unvulcanized rubber tube through a considerably long heating path within the microwave heating device, in order to effect a slow heating of the rubber material. Alternatively, the microwave heating device is turned on and off to effect an intermittent heating operation. In either case, the vulcanization under heat is time-consuming, and the internal structure of the rubber material tends to be excessively vulcanized. Thus, the microwave heating alone is not satisfactory to accomplish the vulcanizing operation in a suitable manner.
On the other hand, dielectric heating by a microwave is not applicable to the production of a rubber hose which has a reinforcing metal layer. Further, the application of the dielectric heating to a rubber hose having a reinforcing fiber layer suffers from a problem. Described more specifically, water or moisture or air contained in the reinforcing fiber layer is vaporized and expanded due to rapid microwave heating thereof, whereby air is trapped at local portions within the unvulcanized rubber material, and at an interface between the rubber material and the reinforcing fiber layer. Consequently, the bonding strength of the fiber layer relative to the rubber layer or tube, and the strength and pressure resistance of the obtained rubber hose are likely to be reduced. Moreover, the air trapped within the rubber hose may significantly deteriorate the appearance and quality of the hose.
Also known is a method as disclosed in German Pat. No. 2715493 (or unexamined published Japanese Patent Application laid open as Publication No. 52-128978), which comprises the steps of: extruding an inner rubber tube on the outer surface of a mandrel made of a metal wire or plastic rod; forming a tubular reinforcing fiber layer made of a nylon or other material, on the outer surface of the inner rubber tube; extruding an outer rubber tube on the outer surface of the reinforcing fiber layer; forming a tubular jacket or cover tube made of a plastic material such as polysulfone, on the outer surface of the outer rubber tube; externally heating the thus-obtained intermediate product by commonly used heating means, to vulcanize the unvulcanized inner and outer rubber tubes; and removing the tubular jacket.
According to the above method, the inner and outer surfaces of the rubber material (inner and outer rubber tubes) are held between the mandrel and the tubular jacket. Therefore, this method eliminates or minimizes air trapping within the hose due to the presence of water or air within the reinforcing fiber layer, and at the same time ensures an intended circular cross sectional shape and a constant thickness of the rubber hose, without employing a conventionally practiced complicated and costly vulcanization technique which employs a continuous sheet of lead. In this respect, the instant method is advantageous.
However, the same method is disadvantageous in that the tubular plastic jacket is first expanded when the intermediate product is subjected to an external heating for vulcanization of the unvulcanized inner and outer rubber tubes. In other words, the tubular jacket is not capable of sufficiently inhibiting expansion of the rubber mass consisting of the inner and outer rubber tubes, during the vulcanization process. As a result, the reinforcing fiber layer of the obtained rubber hose is held merely in contact with the inner and outer rubber tubes, with a very small bonding force between the reinforcing fiber layer and the mating surfaces of the inner and outer rubber tubes. Thus, the obtained rubber hose more or less suffers from a displacement of the inner and outer rubber tubes relative to the reinforcing fiber layer, when the hose is firmly clamped by metal retainers or otherwise pressurized.
In the case where the tubular plastic jacket or cover tube is formed of polysulfone, the obtained rubber hose is difficult to bend, due to relatively high hardness of the polysulfone jacket. If the hose is flattened upon application of an excessive force thereto, the hard plastic jacket prevents the rubber mass from reverting to its original shape. Further, the polysulfone jacket is difficult to remove, at the end of the production process.